This invention relates to novel complexes of macrocyclic polyethers and to a process for isolation of these macrocyclic polyethers from mixtures containing them in which the novel complex is advantageously employed to effect separation of the macrocyclic polyether from the mixture.
Macrocyclic polyethers such as 1,4,7,10,13,16-hexaoxacyclooctadecane have a demonstrated ability to solubilize or complex metal cations in both polar and relatively non-polar media, which, in turn, makes them valuable reagents in a variety of synthesis and separation processes. The disclosure of metal cation complexing properties for these macrocyclic polyethers has stimulated considerable technical interest in their preparation as is evidenced by the following synthesis techniques which have been proposed for 1,4,7,10,13,16-hexaoxacyclooctadecane (also referred to hereinafter by its trival name 18-crown-6).
(1) Elimination of hydrogen chloride from 17-chloro-3,6,9,12,15-pentaoxaheptadecanol, followed by ring closure, in the presence of potassium tert-butoxide, see British patent specification No. 1,285,367. PA1 (2) Catalytic oligomerization of ethylene oxide, see U.S. Pat. No. 3,928,386. PA1 (3) Reaction of triethylene glycol with 3,6-dioxa-1,8-dichlorooctane in the presence of potassium hydroxide and 10% aqueous tetrahydrofuran, as described in J.Org.Chem. 39 (1974)2445-2446. PA1 (4) Reaction of triethylene glycol with bis(2-chloroethyl) ether in the presence of potassium hydroxide and tetrahydrofuran without addition of water, as described in "Synthesis" 1976, 515-516.
While the aforementioned methods for preparing 18-crown-6 show a considerable variation in product yield and cost of starting materials, they all illustrate a disadvantage which is prevalent in previous synthetic techniques for preparing such macrocyclic polyethers in that difficult and costly procedures are required to isolate the product polyethers from the reaction mixture.
The technique disclosed for isolating 18-crown-6 from the reaction mixture obtained by processes (1) and (2) above involves chromatographic separation on acid-washed alumina or silica and elution with readily volatile hydrocarbons. This isolation procedure is not particularly attractive from a commercial standpoint because the adsorption capacities of alumina or silica for the 18-crown-6 are rather low and the used alumina or silica must be regenerated or discarded and replaced by fresh alumina or silica.
In the reference processes (3) and (4) above, potassium chloride and tetrahydrofuran are removed from the reaction mixtures and the resulting product is distilled to afford a crude 18-crown-6 overhead product. Subsequently the distilled 18-crown-6 is mixed with acetonitrile and the mixture obtained is cooled to a very low temperature, e.g., -45.degree. C., to precipitate the 18-crown-6-acetonitrile complex which forms on addition of the acetonitrile. The precipitated complex is then filtered off and the acetonitrile is evaporated from the filtered complex at sub-atmospheric pressure with gentle heating. In the reference process (3), the finished 18-crown-6 is obtained by crystallization from the residue while in reference process (4) the residue is distilled to obtain the 18-crown-6 as a distillate. One disadvantage of using acetonitrile in accordance with reference processes (3) and (4) for isolation of the 18-crown-6 is high solubility of 18-crown-6-acetonitrile complex in acetonitrile at ambient temperature. Because of this high solubility, very low temperatures must be used to precipitate the complex from the excess acetonitrile and even then the complexed 18-crown-6 is obtained in rather low yield. A further disadvantage of reference processes (3) and (4) above is that they both include at least one step wherein the 18-crown-6 is distilled overhead, and therefore, additional measures must be taken to avoid the occurrence of powerful and destructive explosions which are known to occur during the distillation of 18-crown-6, see "Chemical and Engineering News," Sept. 6, 1976, page 5 and Dec. 13, 1976, page 5.
From the foregoing, it is apparent that considerable advantage would be obtained if a simple and cost effective means could be found for isolating macrocyclic polyethers from reaction mixtures which avoids the commercial impracticalities and potential hazards associated with previous separation techniques.